1. Field of the Invention
This invention relates to cellulose fibers, and more particularly, to a method of reducing impurities in cellulose fibers. This invention also discloses the formulations, methods of manufacture and final products of cellulose fiber reinforced cement composite materials using low impurity cellulose fibers.
2. Description of the Related Art
Fiber-reinforced products such as building sheets, panels, planks, and roofing material have been used in the building construction for more than one hundred years. The reinforcement fibers used in such building products include asbestos fibers, cellulose fibers such as those described in Australian Patent No. 515151 and U.S. Pat. No. 6,030,447, the entirety of which is hereby incorporated by reference, metal fibers, and glass fibers and other natural and synthetic fibers. Presently, cellulose is one of the preferred fibers used in most commercial fiber-reinforced building materials because cellulose fiber is an effective, low cost, recyclable natural product compatible with most conventional fiber cement manufacturing processes, including refining and autoclaving.
However, the properties and performance characteristics of most fiber reinforced cement composite materials are highly dependent on the quality of the fibers used. For example, cellulose fibers sometimes contain impurities that can adversely affect the fiber cement composite properties. In particular, harmful organic compounds are sometimes trapped inside the pores and cavities of the cellulose pulp during the pulping process. These organic compounds include lignin and other aromatic components, wood sugar compounds including hextoses (glucose, mannose, and galactose) and pentoses (xylose and arabinose), wood sugar derivatives such as gluconic acid and mannonic acids, fatty acids, resin acids, other organic compounds from wood including extractives and degradation fragments of cellulose, hemicellulose and lignin. In addition to organic compounds, the impurities may also include small amounts of inorganic compounds that are oxidized. These impurities are sometimes collectively referred to as Chemical Oxygen Demand (COD) components.
Each COD component has a certain degree of negative impact on fiber cement reactions, particularly the cement hydration process. The collective effect of all COD compounds released from the pulp in the manufacture of fiber cement composite materials can significantly weaken the bonding between cellulose fibers and other inorganic ingredients in the fiber cement matrix, in which cellulose fibers are typically used as the reinforcement agent. This phenomenon is sometimes referred to as cement poisoning. Moreover, accumulation of the COD impurities released from the pulp can severely contaminate the process water during the manufacture of the fiber reinforced cement composite materials. These adverse effects associated with COD impurities can ultimately result in failure of the final fiber cement products.
To address these problems, most conventional pulp manufacturing processes include a series of cleaning steps that are designed to remove residual chemicals and degraded wood components contained in the pulp. During these cleaning steps, the pulp is typically washed in a series of vacuum, rotary or pressure brown stock washers at a temperature of about 55° C. to 65° C. to remove the residual chemicals from the pulp. However, these processes often fail to remove all COD impurities from the cellulose pulp because of the relatively short retention time and limited washing efficiency. In many cases, a large amount of COD substances remains trapped inside the cavities (lumens) and pores of the fiber cell walls and are carried over to the fiber cement manufacturing processes, which can detrimentally affect the properties of the final product and severely contaminate the process water.
Hence, from the foregoing, it will be appreciated that there is a need for a process that removes substantially all impurities from cellulose fibers during the pulping process. There is also a need for a process of manufacturing low impurity and high performance cellulose fibers for fiber reinforced cement composite materials. To this end, there is a particular need for a cellulose pulp manufacturing process that significantly reduces the amount of COD components in the pulp and can be implemented by using conventional pulp manufacturing equipment.